A typical field effect transistor (FET) comprises a number of layers and they can be configured in various ways. For example, an FET may comprise a substrate, a dielectric, a semiconductor, source and drain electrodes connected to the semiconductor and a gate electrode. When voltage is applied between the gate and source electrodes, charge carriers are accumulated in the semiconductor layer at its interface with the dielectric resulting in the formation of a conductive channel between the source and the drain and current flows between the source and the drain electrode upon application of potential to the drain electrode.
FET's are widely used as a switching element in electronics, for example, in active-matrix liquid-crystal displays, smart cards, and a variety of other electronic devices and components thereof. The thin film transistor (TFT) is an example of a field effect transistor (FET). The best-known example of an FET is the MOSFET (Metal-Oxide-Semiconductor-FET), today's conventional switching element for high-speed applications. Presently, most thin film devices are made using amorphous silicon as the semiconductor. Amorphous silicon is a less expensive alternative to crystalline silicon. This fact is especially important for reducing the cost of transistors in large-area applications. Application of amorphous silicon is limited to low speed devices, however, since its maximum mobility (0.5-1.0 cm2/V.sec) is about a thousand times smaller than that of crystalline silicon.
Although amorphous silicon is less expensive than highly crystalline silicon for use in TFT's, amorphous silicon still has its drawbacks. The deposition of amorphous silicon, during the manufacture of transistors, requires relatively costly processes, such as plasma enhanced chemical vapor deposition and high temperatures (about 360° C.) to achieve the electrical characteristics sufficient for display applications. Such high processing temperatures disallow the use of substrates, for deposition, made of certain plastics that might otherwise be desirable for use in applications such as flexible displays.
In the past two decades, organic materials have received significant attention as a potential alternative to inorganic materials, such as amorphous silicon, for use in semiconductor channels of FET's. Compared to inorganic materials, that require a high-temperature vacuum process, organic semiconductor materials are simpler to process, especially those that are soluble in organic solvents and, therefore, capable of being applied to large areas by far less expensive processes, such as roll-to-roll coating, spin coating, dip coating and micro-contact printing. Furthermore organic materials may be deposited at lower temperatures, opening up a wider range of substrate materials, including plastics, for flexible electronic devices. Accordingly, thin film transistors made of organic materials can be viewed as a potential key technology for plastic circuitry in display drivers, portable computers, pagers, memory elements in transaction cards, and identification tags, where ease of fabrication, mechanical flexibility, and/or moderate operating temperatures are important considerations. However, to realize these goals, OFET semiconductor and dielectric components should ideally be easily manufactured using high-throughput, atmospheric pressure, solution-processing methods such as spin-coating, casting, or printing.
To date in the development of organic field effect transistors (OFET's) considerable efforts have been made to discover new organic semiconductor materials and optimizing properties of such materials. These efforts have been quite fruitful and a number of organic semiconducting materials have been designed and, to a lesser extent, structure-property relationships of such materials have been studied.
Accordingly, fused acenes such as tetracene and pentacene, oligomeric materials containing thiophene or fluorene units, and polymeric materials like regioregular poly(3-alkylthiophene) have been shown to perform in OFET's as “p-type” or “p-channel,” semiconductors, meaning that negative gate voltages, relative to the source voltage, are applied to induce positive charges (holes) in the channel region of the device. Examples of acene and heteroacenes based semiconductors are well known in the prior art.
As an alternative to p-type organic semiconductor materials, n-type organic semiconductor materials can be used in FET's where the terminology “n-type” or “n-channel” indicates that positive gate voltages, relative to the source voltage, are applied to induce negative charges in the channel region of the device. For example, n-type semiconductors based on diimide materials are known in the art.
The overall performance of an OFET is dependent on a number of factors such as the degree of crystallization and order of organic semiconductor layer, charge characteristics, and trap density at the interfaces between dielectric and organic semiconductor layers, and the carrier injection ability of the interfaces between source/drain electrodes and organic semiconductor layers. Although, the gate dielectric layer is intended to ensure a sufficiently good electrical insulation between the semiconductor and the gate electrode, it plays an important role in the overall performance of an OFET. In particular, the gate dielectric permits the creation of the gate field and the establishment of the two-dimensional channel charge sheet. Upon application of a source-drain bias, the accumulated charges move very close to the dielectric-semiconductor interface from the source electrode to the drain electrode.
Since the charge flow in organic semiconductor occurs very close (˜1 nm) to the dielectric interface, it is important to optimize chemical and electrical behavior of the dielectric layer. Besides these factors, the dielectric surface morphology has a great effect on carrier or charge mobility of the semiconductor. The surface morphology of the dielectric material and variations in its surface energies [for example, surface treatment via self-assembled monolayers (SAMs)] have been shown to modify the growth, morphology, and microstructure of the vapor/solution-deposited semiconductor, each of these being a factor affecting mobility and current on/off ratio, the latter being the drain-source current ratio between the “on” and “off” states, another important device parameter. The properties of the dielectric material can also affect the density of state distribution for both amorphous and single-crystal semiconductors.
Gate dielectric materials for OFET's can be divided into inorganic and organic materials. Inorganic dielectric materials like silicon oxide (SiO2), silicon nitride (SiNx), aluminum oxide (AlOx), and tantalum oxide (TaOx) are conventionally deposited via chemical vapor deposition (CVD) and plasma enhanced CVD methods which are high temperatures (>300° C.) processes and not compatible with polymeric substrates. Lower processing temperatures usually lead to poor quality films with pinholes, resulting in poor insulating properties. As a result it is necessary to use thick layers (more than 100 nm) to ensure sufficiently good insulator properties which results in increased supply voltages for operation of such circuits. Another widely used process is ion beam deposition, but it needs high vacuum and expensive equipment that are incompatible with the goal of very low cost production. Similarly, use of other high dielectric constant inorganic materials as barium zirconate titanate (BZT) and barium strontium titanate (BST) need either a high firing temperature (400° C.) for the sol-gel process, or radiofrequency magnetron sputtering, which also requires vacuum equipment, and can also have stoichiometric problems.
In addition to higher temperature processing, inorganic insulating layers generally require interfacial modification before they can be used with an organic semiconductor. It has been shown that the presence of polar functionalities (like —OH groups on SiO2 surface) at the dielectric-organic semiconductor interface trap charges which results in lowers carrier mobility in organic semiconductors. This is especially true for n-type organic semiconductors and OFET devices comprising n-type semiconductors. For example, a silicon dioxide dielectric surface is commonly functionalized with long alkyl chain silanes [commonly octadodecyl trichlorosilane (OTS)] using a solution phase self assembly process. This results in a low energy dielectric surface with very few chemical defects or reactive functionalities that could adversely affect the OFET device performance.
Most organic materials used in OFET's cannot withstand the high processing temperatures used with conventional inorganic materials. For example, the 200° C. or higher temperatures needed to process conventional inorganic materials would at the very least cause a polymeric substrate to deform, and might cause further breakdown of the polymer or even ignition at high enough temperatures. Deformation is highly undesirable, since each layer of the structure has to be carefully registered with the layers below it, which becomes difficult to impossible when the layers below it are deformed due to processing temperatures.
As an alternative to inorganic gate dielectrics, insulating polymers have been proposed for fabrication of OFET's. Polymers generally have the advantage that they can be processed at relatively low temperatures below 200° C. However, compared to inorganic dielectrics, the insulating property of thin layers of polymeric dielectrics is usually poor on account of leakage currents. Hence, comparatively thick layers (more than 100 nm) of polymeric dielectrics are usually employed in fabrication of OFET's. As a consequence, integrated circuits having OFET's with polymeric gate dielectrics require the use of comparatively high supply voltages. In pentacene layers deposited on polymeric dielectrics, the mobility of the charge carriers is similar or higher in comparison with inorganic dielectrics.
A number of polymers have been used as gate dielectrics in OFET's. Halik et al. (Journal of Applied Physics 93, 2977 (2003)) describe the use of poly(vinyl phenol) (PVP) that is thermally cross-linked with polymelamine-co-formaldehyde as a gate dielectric layer to make p-type OFET's. However, this attempt is limited in usefulness since a high temperature of about 200° C. is required to attain crosslinking. Similarly, U.S. Patent Application Publication 2010-0084636 (Lin et al.) describes a photosensitive dielectric material comprising a poly(vinyl phenol) based polymer, a crosslinking agent, and a photoacid generator. However, the presence of acid is not desirable since it could have deleterious effect on the performance of OFET's.
U.S. Pat. No. 7,298,023 (Guillet et al.) describes the use of organic insulator (or dielectric) comprising a base copolymer of PVDC-PAN-PMMA having the general formula (—CH2Cl2—)x—(—CH2CH(CN)—)y—(CH2C(CH3)(CO2CH3))z, wherein x, y, z, in each case (independently from one another) can assume values between 0 and 1, for use in OFET's and organic capacitors. However, the presence of polar groups at the dielectric interface creates dipolar disorder that lowers the carrier mobility.
U.S. Patent Application Publication 2008-0283829 (Kim et al.) discloses an organic insulator composition comprising a crosslinking agent and a hydroxyl group-containing oligomer or hydroxyl group-containing polymer. However, the presence of hydroxyl groups at the organic semiconductor gate dielectric interface is not desirable as hydroxyl trap charges.
U.S. Pat. No. 6,232,157 (Dodabalapur et al.) discloses the use of a polyimide as material for organic insulating films. U.S. Pat. No. 7,482,625 (Kim et al.) discloses a thermosetting composition for organic polymeric gate insulating layer in OFET's. U.S. Pat. No. 7,482,625 also describes blending polyvinyl phenol with another polymer in consideration of physical, chemical, and electrical characteristics. The polymers that can be blended include polyacrylates, poly(vinyl alcohol), polyepoxys, polystyrene, and poly(vinyl pyrrolidone). U.S. Pat. No. 7,741,635 (Kim et al.) describes photo-crosslinkable polymer dielectric composition comprising an insulating organic polymer such as poly(methyl methacrylate) (PMMA), poly(vinyl alcohol) (PVA), poly(vinyl pyrrolidone) (PVP), or poly(vinyl phenol) (PVPh) and a copolymer thereof, a crosslinking monomer having two or more double bonds, and a photoinitiator. U.S. Patent Application Publication 2008-0161464 (Marks et al.) discloses a crosslinked polymeric composition as gate dielectric material.
EP 1,679,754A1 (Kim et al.) describes coating a surface of a crosslinked poly(vinyl phenol) gate dielectric with a thin film of fluorine containing polymer. Although OFET device performance may improve in the presence of fluorine containing polymer, the process requires undesirably coating multiple polymer layers. U.S. Pat. No. 7,352,038 (Kelley et al.) describes an OFET comprising a substantially nonfluorinated polymeric layer interposed between a gate dielectric and an organic semiconductor layer.
U.S. Pat. No. 7,528,448 (Bailey et al.) describes a multilayer thermal imaging dielectric donor composition of a dielectric layer comprising one or more dielectric polymers such as acrylic and styrenic polymers and heteroatom-substituted styrenic polymers.
WO2007-129832 (Lee et al.) describes a composition for forming a gate insulating layer of an OFET comprising an acrylate polymer and show mobilities in the range of 0.19-0.25 cm2/V.sec, which are significantly lower than those reported for poly(methyl methacrylate) dielectric compositions.
While a number of dielectric compositions and materials have been proposed for uses in OFET devices, polymer dielectric materials that work well in p-type or p-channel OFET's usually do not necessarily perform as well with OFET's comprising n-type semiconductors. It has been proposed that the presence of reactive chemical functionalities and dipoles at the semiconductor-polymer dielectric interface have much more significant effect on n-type semiconductors than p-type semiconductors. U.S. Pat. No. 7,638,793 (Chua et al.) describes that for an n-channel or ambipolar OFET the organic gate dielectric layer forming an interface with the semiconductive layer; should have less than 1018 cm−3 bulk concentration of trapping groups, and the use of poly(siloxanes) (for example Cyclotene® electrical polymer), poly(alkenes), and poly(oxyalkylenes) as dielectric materials.
Although various polymer dielectric compositions are known, a number of problems still remain in terms of the process of making such dielectric layers and improving overall performance in OFET's. As discussed before, some the polymer dielectric compositions require coating of multiple layers that is a difficult and costly process. Other examples of dielectric compositions include thermosetting polymers comprising poly(vinyl phenol) as the main component and require a high temperature annealing and crosslinking process. It is difficult to crosslink all phenolic groups during thermal annealing and thus the presence of phenolic groups in dielectric is not desirable.
Furthermore, most polymer materials are usually not appropriate for use in low voltage applications due to their low dielectric constant (k). There have been many attempts to obtain a high capacitance with gate dielectrics by reducing their thickness or using polymer-inorganic composites to increase the dielectric constant (k) to produce low voltage operating OFET's.
For example, Japanese Patent 3,515,507 (Aoki et al.) discloses an organic polymer and an inorganic material that are mixed to provide insulating film with flexibility and high dielectric constant. In accordance with this reference, a powder obtained by mechanically grinding a ferroelectric material such as barium titanate is dispersed in an organic polymer to compensate the dielectric constant of the resulting insulated gate film and hence lower the gate voltage required for the operation of OFET. However, when this method is used, the thickness of the insulating film is limited to the size of the inorganic material thus ground. Furthermore, since a solid material is dispersed in an organic polymer solution, an uneven dispersion is formed, possibly causing the generation of local electric field and concurrent dielectric breakdown during the operation of transistor. Importantly, since the inorganic material is merely present in the organic polymer and thus does not compensate the chemical resistance of the insulating film, the resulting insulating film cannot be subjected to any processes involving the use of solvents.
Japanese Patent Publication 2003-338551 (Shindo) discloses a technique of forming a thin ceramic film as an insulating film on the surface of silicon wafer by a sol-gel method allowing a low temperature treatment. The resulting thin ceramic film can be prevented from being cracked, making it possible to efficiently produce electronic parts having a high reliability. However, the thin ceramic film is an insulating film made of an inorganic material that can be applied to silicon wafer, which is nonflexible and hard, but it cannot be applied to flexible substrates.
U.S. Patent Application 2010-0230662 (Chen et al.) discloses gate insulating layer comprising an azole-metal complex compound. However, pentacene based OFET having an azole-metal complex compound as a dielectric shows lower mobility and poor current on/off ratio.
U.S. Patent Application 2010-0051917 (Kippelen et al.) discloses embodiments of OFET's having a gate insulator layer comprising organic polymer, specifically poly(vinyl phenol) (PVP), nanocomposites incorporating metal oxide nanoparticles (for example, barium titanate (BaTiO3), strontium titanate (SrTiO3), and barium zirconium titanate (BaZrTiO3)) coated by organic ligands and methods of fabricating such OFET's.
Usually dispersions of metal oxide nanoparticles in a polymer matrix are not homogeneous. In an attempt to obtain a uniformly dispersed organic-inorganic mixture system as an ordinary material technique, it has been practiced to prepare a composite film from a mixture of a solution of metal alkoxide that is a precursor of inorganic oxide and an organic polymer solution by a sol-gel method. In this case, it is expected that as the dispersion of organic polymer is made more on monomolecular level, the thermal stability of the organic polymer is more enhanced. Thus, when a polymer or molecules capable of making hydrogen bond such as hydroxyl group or electrostatic mutual action are present in a metal alkoxide solution, a sol-gel polycondensation is formed selectively on the surface of the compound to form a dried gel.
A composite film comprised of poly(methyl methacrylate-co-methacrylic acid) (PMMA-co-MAA)/sol-gel-derived TiO2 has been used as gate dielectric layer in pentacene based OFET's. However, surface roughness of such films is quite high (about 2.1 nm). In the case of deposition of small semiconductor molecules, such as pentacene, molecular orientation and grain morphology depend strongly on the surface roughness and energy of the underlying film. Surface smoothness of poly(4-vinylphenol)-composite is better (about 1.3 nm) and pentacene OFETs show smaller threshold voltage (Kim et al. J. Am. Chem. Soc. 132, 14721, 2010).
Although various polymer dielectric compositions are known, a number of problems still remain in terms of the process of making such dielectric layers and improving overall performance in OFET's. As discussed before, to increase the dielectric constant of polymer dielectric materials, a number of polymer-inorganic composites, by adding metal oxide nanoparticles and making metal oxide nanoparticles by sol-gel method, are known. However, these methods do not result in crosslinking of the dielectric layer, which is a key requirement in solution processable OFET's. To achieve crosslinking photocurable resins have to be used.
Since most of the polymer dielectrics known in prior art are not crosslinked, the resulting dielectric film cannot be subjected to any processes involving the use of solvents. To address this problem various cross linked polymer dielectrics have been developed for example as described by Halik et al. [Journal of Applied Physics 93, 2977 (2003)]. In addition, U.S. Pat. No. 7,482,625 (Kim et al.) describes blending polyvinyl phenol with another polymer for certain physical, chemical and electrical characteristics. However, this approach has limited application since a high temperature of about 200° C. is required to attain crosslinking.
Thus, there is a need for polymer dielectric materials that are soluble in environmentally friendly solvents, easy to apply as a single layer, that exhibit good electrical and insulating properties, and that can be prepared from commercially available polymer or molecular precursors using solution processes at low temperatures and atmospheric pressures. It is also desired that they have higher dielectric constant (k) and could be thermally and/or photochemically crosslinked. It is difficult to find polymeric materials that have all of these properties because some polymers will exhibit improvements in some of the properties but exhibit worse effects in others.
With the difficulty in balancing all desired properties in mind, there continues to be research to find useful polymeric dielectric materials.